Clarifiers are well-known in the prior art for separating suspended solids from clarified liquor. Most clarifiers operate by sedimentation of solids (i.e. solids sink and are collected from a bottom portion of the clarification vessel) or flotation (i.e. solids are caused to float and are removed as a flotation blanket from the surface of the clarification vessel). Conventional dissolved air flotation (DAF) clarifiers are not typically used in applications where secondary quality effluent is desired, especially in plants having high flow rates. This is primarily due to the substantial amount of energy required for the pressurization of air and recycle water in the reaction chamber. However, in applications where DAF clarifiers have been used, it has been shown that particle sizes from 0.5-500 microns can be easily separated from the mother liquor by flotation whereas sedimentation clarification is limited to particle sizes of roughly 50-500 microns. FIG. 1, which is derived from a Water Environment Federation Manual of Practice1, compares various treatment technologies for separating wastewater organics from an influent stream, including sedimentation and flotation clarification. 1Manual of Practice 8, Design of Municipal Wastewater Treatment Plants, (1991, Vol. I, p. 46)
In practice, most DAF clarifiers are used in applications where high quality effluent is not required. For example, DAF units processing high influent solids concentrations have been used extensively in low flow applications, such as sludge thickeners. Such thickeners produce effluent quality in the 100 mg/L and greater total suspended solids (TSS) range, even with the use of high doses of polymer. DAF units have also been used successfully in high flow installations where the influent solids concentration is relatively low. However, very few DAF units are known in the prior art that can process high concentrations of influent solids and also produce high quality effluent without the use of polymers, particularly when such units are loaded at conventional clarifier design rates.
The flotation separation of biological solids has been successfully achieved by the inventor using deep vertical shaft bioreactors as described in U.S. Pat. Nos. 5,645,726 and 5,650,070. In deep vertical shaft bioreactors, the entire mixed liquor is subjected to pressure and hence dissolved gas forms not only in the liquid around the biomass particle but also within the cell wall of the microbes. This makes a portion of the sludge biomass buoyant for a period of time until the gas concentrations equilibrate across the cell wall. Such bio-flocculent floats faster and forms a thicker float blanket than flocculent attached to the surface of the gas bubbles. The float blanket can, however, be relatively fragile and hence care must be taken in separating the activated sludge from the clarified subnatent.
Most prior art clarifiers exhibit shortcomings in handling fragile flocculent. For example many prior art systems, such as described in U.S. Pat. Nos. 4,279,754 and 5,330,660 employ sloped stationary beaches, the highest point of which, is elevated slightly above the fill level of liquid in the clarifier bowl. At the back of the beach is a sludge collection trough that collects the sludge that is pushed by scrapers onto the sloped beach, up the ramp, and into the trough. The top of the trough is slightly higher than the liquid level in the clarifier to prevent return activated sludge (RAS) flow between scraper discharges. Sludge removal therefore depends on mechanically pushing the sludge up the sloped beach and into the trough. This action requires a relatively highly concentrated (i.e. thick) sludge, a tight scraper-to-beach fit and a mechanically strong sludge float blanket to form in front of the scraper. The sludge float blanket usually needs to be strengthened by the use of polymers.
Thus, in conventional designs, polymers are used primarily to compensate for mechanical design limitations, such as aggressive manipulation of the biomass using scrapers, ramped beaches and elevated RAS troughs protruding above the liquid fill level. Ideally, because of cost, the use of polymers should be eliminated or used only sparingly in secondary flotation clarifiers. However, if use of polymers is limited, the flocculated float solids that form at the surface of the liquid are relatively more fragile than floc formed in sludge thickeners using high dosages of polymer.
Krofta has developed various improvements to flotation clarifier design, as exemplified by U.S. Pat. No. 6,174,434. For example, Krofta recognized the importance of rampless beaches and gravity RAS troughs when handling fragile sludge blankets. However, the Krofta process described in the '434 patent includes the step of dipping a scoop into the sludge, thus disturbing the float, and mechanically elevating the scoop so that the sludge will flow by gravity. While this approach is an ingenious improvement over prior designs, the Krofta flocculent handling assembly is mechanically complex and inefficient.
Most flotation clarifiers described in the prior art are rectangular in construction. One of the critical parameters in rectangular clarifier design is to maximize the width of the beach and the length of the overflow weir. For example, the overflow weir in a rectangular clarifier may be double-sided or trough-shaped to maximize its length. The same design principles apply in the case of circular clarifiers. However, circular clarifiers can exhibit significant process advantages especially when built on a large scale (e.g. 60 feet in diameter or more). For example, large diameter circular clarifiers provide an opportunity to place the overflow weir along the peripheral wall of the clarifier to thereby maximize its length. Rectangular clarifiers suffer from short beach lengths and consequently decreased ability to return solids such as return activated sludge. In circular clarifiers, beaches may be deployed radially to provide a larger effective surface for sludge collection. Also, in circular clarifiers the influent feed may be introduced from a central, inner portion of the clarifier and effluent may be removed from the outer perimeter under relatively quiescent conditions.
The need has therefore arisen for an improved circular flotation clarifier capable of achieving a high degree of separation of float solids or other separable matter at commercially practical loading rates, without the use of polymers and without damaging fragile flocculent.